Online communities are new social structures dependent on modern information technology, and they face equally modern challenges. Although satisfied members regularly consume content, it is considerably harder to coax them to contribute new content and help recruit others because they face unprecedented social comparison and criticism. We propose that engagement-a concept only abstractly alluded to in information systems research-is the key to active participation in these unique sociotechnical environments. We constructed and tested a framework that demonstrates what engagement is, where it comes from, and how it powerfully explains both knowledge contribution and word of mouth. Our results show that members primarily contribute to and revisit an online community from a sense of engagement. Nonetheless, word of mouth is partly influenced by prior satisfaction. Therefore, engagement and satisfaction appear to be parallel mediating forces at work in online communities. Both mediators arise from a sense of communal identity and knowledge self-efficacy, but engagement also emerges from validation of self-identity. Nevertheless, we also found signs that the contributions of the most knowledgeable users are not purely from engagement, but also from a competing sense of self-efficacy. Our findings significantly contribute to the area of information systems by highlighting that engagement is a concrete phenomenon on its own, and it can be directly modeled and must be carefully managed.